Electrorheological fluids, also called electroviscous fluids, have long been known (see Duff, A. W., Physical Review, 4, (1), 23 (1896)). Early studies were related to pure liquids with poor Theological effects. Later, electrorheological dispersions attracted attention, which resulted in considerable electrorheological effect.
The electrorheological effect was attributed by Klass (Klass, D. L. et al., J. of Applied Physics, 3, (1), 67 (1967)) principally to the induced polarization of the double layers around the dispersed particles in an electric field. The ions adsorbed by the dispersed particles (of silica gel, for example) are uniformly distributed when the external electric field is zero, but are displaced and interact electrostatically with each other when an electric field is applied. Thus bridges are formed by the particles between the electrodes, which is responsible for shear resistance against any stress applied to the fluid, or the electrorheological effect.
Winslow proposed an electrorheological fluid consisting of paraffinic hydrocarbons, silica gel powder, and water as the polarizing agent (Winslow, W. M., J. of Applied Physics, 20, 1137 (1949). This study has prompted to refer to the electrorheological effect as the Winslow effect. Such fluids containing solid particles as the disperse phase have originally had a problem in the dispersibility of the disperse phase, resulting in dense precipitate after a long standing period, or gel formation in several minutes to several hours at temperatures about 100.degree. C., thus losing the function of electrorheological fluid.
Aiming at use in vessels using rubber components, Japanese Patent Application Laid-Open Nos.: 140581/1993, 348193/1992, 299893/1989 and 304144/1989 disclose electrorheological fluids consisting of organopolysiloxanes as the dispersion medium, and fine silica particles surface-modified by specific compounds as the disperse phase. For example, Japanese Patent Application Laid-Open No. 304144/1989 discloses the surface treatment of fine silica particles with either EQU X--Si--(OR).sub.3 ( 1)
or EQU (RO).sub.3 --Si--X--Si--(OR).sub.3 (2)
However, surface modification with compounds containing saturated hydrocarbyl, unsaturated hydrocarbyl, aromatic hydrocarbyl or halohydrocarbyl as X in (1) or (2) above does not enhance the affinity of said particles with organopolysiloxanes as the medium, leading to formation of precipitate that cannot easily be redispersed after a long standing period.
1. Disclosure of the Invention
The present invention aims at providing an electrorheological fluid comprising fine silica particles dispersed in a nonconducting liquid, specifically an organopolysiloxane, which have excellent dispersion stability, a low initial viscosity, and enhanced electrorheological effect.
2. Means to solve the Problems
In an attempt to develop electrorheological fluids with such favorable features as stated above, the inventors have found that fine surface-modified silica particles obtained by esterification with an alcohol-modified silicone oil as the disperse phase give an electrorheological fluid with excellent long-term stability of dispersion, a low initial viscosity and enhanced electrorheological effect, thus solving all the problems in the development of electrorheological fluids. The present invention was accomplished on the basis of this discovery.
Thus, the present invention provide an electrorheological fluid comprising a fine silica particles with the surface esterified with an alcohol-modified silicone oil (referred to as "surface-modified fine silica particles" hereinafter) dispersed in a nonconducting fluid.
The invention also provides, as preferred embodiments, electrorheological fluids (1)-(6) below.
(1) An electrorheological fluid in which said nonconducting fluid is a silicone oil, with which surface-modified fine silica particles are compounded,
(2) An electrorheological fluid in which said nonconducting fluid is an alkylbenzene and/or a mineral oil, with which surface-modified fine silica particles are compounded,
(3) An electrorheological fluid consisting of a nonconducting fluid compounded with surface-modified fine silica particles and a polarizing agent,
(4) An electrorheological fluid in which said nonconducting fluid is a silicone oil, with which surface-modified fine silica particles and a polarizing agent are compounded,
(5) An electrorheological fluid in which said nonconducting fluid is an alkylbenzene and/or a mineral oil, with which surface-modified fine silica particles and a polarizing agent are compounded, and
(6) An electrorheological fluid consisting of a nonconducting fluid compounded with surface-modified fine silica particles which have 0.2 bonds/nm.sup.2 -8 bonds/nm.sup.2 with the alcohol-modified silicone oil.
3. Advantages of the Invention
The electrorheological fluids according to the invention have excellent long-term stability of the dispersion of solid particles, a low initial viscosity, and highly improved electrorheological effect, since the fine silica particles with the surface esterified with an alcohol-modified silicone oil have high affinity with the nonconducting fluid, specifically organopolysiloxanes. These features eliminate problems which have been regarded as inherent to electrorheological fluids and enable long-term, stable applications for electric control systems that respond promptly to external forces in devices under constant vibration, such as variable dumpers and engine mounts.